Exemplary thermoelectric generators and the principles of operation thereof have been described in U.S. Pat. Nos. 3,404,036 and 4,094,877, the disclosures of which are hereby expressly incorporated by reference. "Sodium heat engine" is the name commonly given to such thermoelectric generators which electrochemically expand sodium metal across the solid electrolyte. While other alkali metals may be employed in the generators of this invention, the sodium heat engine is described herein exemplary of these generators.
A sodium heat engine generally comprises a closed container separated into a first and second reaction zone by a solid electrolyte. Liquid sodium metal is present in the first reaction zone (i.e., on one side of the solid electrolyte) and the first reaction zone is maintained during operation of the device at a pressure higher than that of the second reaction zone. In the lower pressure second reaction zone, an electrically conducting permeable electrode is in contact with the solid electrolyte. During operation of the sodium heat engine, a heat source raises the temperature of the liquid sodium within the first reaction zone to a high temperature (T.sub.2) and a corresponding high vapor pressure (P.sub.2) which creates a sodium vapor pressure differential across the solid electrolyte. In response to this pressure differential, elemental sodium gives up electrons to an electrode in contact with the sodium metal and the resulting sodium ions migrate through the solid electrolyte. The electrons, having passed through an external load, neutralize sodium cations at the permeable metal electrode-solid electrolyte interface. Elemental sodium metal evaporates from the permeable electrode and migrates through the low pressure (P.sub.1) second reaction zone, i.e., vacuum space to a low temperature (T.sub.1) condenser. The condensed liquid sodium may then be returned back to the higher temperature reaction zone within the first reaction zone, e.g., by means of a return line and an electromagnetic pump, to complete a closed cycle. Thus, during operation of the device, sodium passes from the first reaction zone to the second and, where the device include means for returning the sodium back to the first reaction zone, the sodium completes the cycle. The process occurring in the electrolyte and at the sodium-electrolyte and electrode-electrolyte interfaces is nearly equivalent to an isothermal expansion of the alkali metal from pressure P.sub.2 to P.sub.1 at the temperature T.sub.2. No mechanical parts need move, and the work output of the process is electrical only.
In order to draw electrical power from these generators, it is necessary to bring an electrical conductor wire from the permeable electrode in the low pressure space to an external load, i.e., the conductor wire must pass through the wall defining the low pressure space. A hermetically sealed feedthrough structure is employed to bring the conductor wire through the wall of the thermoelectric generator and electrically insulate the conductor wire from the wall which is generally not at the same potential as the wire. The feedthrough structure must be corrosion resistant to hot alkali metal and its vapors and any seals employed therein must remain hermetic in spite of the thermal cycling to which they are subjected during operation of the generator. Common to such feedthroughs is the use of an insulator which is dense, refractory, and sufficiently inert to hot alkali metal, such as dense pure aluminum oxide. Metal components, made of corrosion resistant metal such as stainless steel, may also be employed in combination with the insulator in electrical feedthroughs. In order to minimize the stresses in the feedthrough seals joining the insulator with the metal components, it is desirable to have a good match in the thermal expansivities of the metal and insulator components. Seals of mis-matched components are relatively susceptible to failure from thermal cycling. A simple conventional feedthrough may comprise a short length of insulator tube sealed into an opening in the cell wall. The wire passes through the opening in the insulator tube and a braze material is used to seal the tube.
Sharma in U.S. Pat. No. 3,847,675 discloses the use of a ceramic sheath of Al.sub.2 O.sub.3 surrounding an electrical lead as it passes through the casing of a molten salt battery. The sheath is attached to the surface of the conductor wire interiorly and exteriorly of the bottom wall. U.S. Pat. No. 3,005,865 to Jonssen discloses a metallic tube surrounding the conductor as it passes through the battery wall having a tubular insert of glass between the metallic element and the conductor for insulation purposes. Dinin in U.S. Pat. No. 1,379,854 discloses a sleeve of or tube closely surrounding the conductor as it passes through the wall wherein the tube is formed of a celluloid material. Callender in U.S. Pat. No. 687,121 shows an insulating tube concentrically arranged around the battery and interposed between it and a carbon cylinder.
Different portions of the walls defining the low pressure space can be either hot (700.degree.-1000.degree. C.) as when they are coupled to the heat source, or cool (100.degree.-500.degree. C.) as when they are part of the condenser or heat sink. Locating a feedthrough at a "cool" wall is advantageous in that less stress is placed on the metal-insulator seals when using feedthroughs of conventional design. However, alkali metal vapor can condense to form an electrically conductive liquid film bridging across the insulator from the conductive wire to the wall. Since the conductor is at the positive potential of the electrode and the wall is usually at the negative potential of the cell, this liquid film will short out the cell. On the other hand, locating a feedthrough at a "hot" wall assures that no liquid alkali metal will condense across conventional insulator elements to cause a short circuit, but due to the higher temperatures the feedthrough components are then subjected to greater thermal stress and corrosion by both air and sodium vapor.